A ring transmission system of BLSR (Bi-directional Line Switched Ring) structure having a bidirectional transmission capability (hereafter, may simply be referred to “BLSR system” depending on the cases) is realized conforming to the SONET (Synchronous Optical Network), in which, typically, a ring-shaped transmission path is configured by connecting between each plurality of optical transmission apparatuses through optical fiber lines having work channels ordinarily for use in data transmission and protection channels for standby. In normal cases, a data is transmitted from an add node in which the data is inserted into the ring to a drop node in which the data is extracted from the ring, through a path being set in the work channel. On the occurrence of any fault in the work channel, the data transmission path is switched over so as to use the protection channel, through which the data is circulated in the opposite direction to the transmission direction through the work channel, and the data is transmitted to the drop node.
Here, as a transmission mode in the BLSR system, a system termed OC (Optical Carrier)-48 is adopted. In the OC48, in a line configured of one optical fiber, 48 time slots are time multiplexed, thereby forming 48 channels per line, of which a half, i.e. 24 channels, are allotted to the work channels, while the remaining 24 channels are allotted to the protection channels. In a so-called two-fiber BLSR structure, each node is connected with two optical fibers, thus configuring a ring network, in which each optical fiber line includes 24 work channels and 24 protection channels. Further, the transmission directions of the respective optical fibers are opposite. Namely, the work channels and the protection channels of one optical fiber transmit data clockwise, while the work channels and the protection channels of the other optical fiber transmit data counterclockwise. Therefore, on the occurrence of a fault in a certain work channel, the data transmission path is switched over to a protection channel of the other optical fiber, and data are transmitted in the opposite direction to the original transmission direction through the work channel.
FIG. 1 shows a diagram illustrating an exemplary operation of the BLSR ring transmission system. In FIG. 1, solid lines represent the work channels, while dotted lines represent the protection channels. The above is also applicable to the drawings following FIG. 1, FIG. 1(a) illustrates an operation example in a normal case, while FIG. 1(b) illustrates an operation example in case a fault has occurred. The BLSR system includes four nodes 1, 2, 3 and 4, each node being connected through two optical fibers, so as to configure a ring network. Further, each optical fiber includes work channels and protection channels. Also, each node includes an E (East) side and a W (West) side, where a clockwise rotation direction is defined as E→W direction, while a counterclockwise rotation direction is defined as W→E direction.
In FIG. 1, when a data is transmitted from an add node 2 to a drop node 4, a path is set on a work channel of the optical fiber via node 1 through which the data is transmitted counterclockwise (W→E direction). In the normal case shown in FIG. 1(a), according to the above path, data is transmitted from add node 2 via node 1, and reaches drop node 4. Meanwhile, when there is any fault on the work channel, for example, between node 1 and node 4 as shown in FIG. 1(b), the transmission line is switched in node 1 to a protection channel in the other optical fiber, and the data is transmitted through the protection channel in the clockwise direction (E→W direction) via nodes 2, 3, and reaches drop node 4.
Now, in the BLSR system, when the path is set on a work channel in the ring, as information for deciding whether data can be relieved in the event of a fault on the work channel of interest, a squelch table is generated on a channel-by-channel basis in each node in which the path exists. ‘Squelch’ signifies processing for preventing the data (optical signal) being in transmission from being connected to an incorrect node and relieving the data, which is performed when the ring transmission path falls into a state of being divided into sections due to the fault occurrence on a ring line. More specifically, the squelch is the processing performed in each node to add an AIS (Alarm Indication Signal) to the optical signal, thereby preventing optical signal crosstalk.
To realize the above squelch function, the squelch table is provided in each node. The squelch table is a table having cross-connect information necessary for squelching stored therein. More specifically, for each work channel, data indicating a source node (add node) ID and a destination node (drop node) ID are stored. By means of the above squelch table, each node can obtain cross-connect information, regarding from which node to which node a path has been set.
FIG. 2 shows a diagram illustrating an exemplary squelch table format. As shown in FIG. 2, a squelch table (SQLTBL) retains information of the add node and the drop node in regard to a path, on a basis of each cross-connect direction (W→E direction/E→W direction) and each node side (E side/W side). Each squelch table is generated in the entire nodes in which a path is existent.
FIG. 3 shows a diagram illustrating an exemplary squelch table (SQLTBL) being set by a path exemplified in FIG. 1. There are shown the examples of the squelch tables for node 2 in FIG. 3(a), for node 3 for FIG. 3(b), and for node 4 in FIG. 3(c). Node 2 is an add node, in which optical signals (data) are inserted from the E side of node 2, and the data are transmitted in the W→E direction, and therefore, IDs of the add node and the drop node are stored in the fields on the E side of the W→E direction, as shown in FIG. 3(a). Node 1 is a through node, in which optical signals (data) are transmitted on both W side and E side of node 1, and therefore, IDs of the add node and the drop node are stored in the both fields of the E side and the W side in the W→E direction. Node 4 is a drop node, in which optical signals from node 1 are received on the W side. Accordingly, as shown in FIG. 3(c), IDs of the add node and the drop node are stored in the fields of the W side in the W→E direction.
The above squelch table is automatically formed at the time of setting the cross-connect, by use of a squelch data link, which is a control channel.
FIG. 4 shows an exemplary format of a squelch data link. The squelch data link includes cross-connect information constituted of the combination of the ID (SRC_ID) of the add node (source node) with the ID (DST_ID) of the drop node (destination node). Further, the squelch data link includes areas for storing the cross-connect information on the basis of ‘for transmission’/‘for reception’, W side/E side, and transmission direction (E→W direction/W→E direction). Each cross-connect information set is formed of 1 byte: 4 bits assigned to the add node ID, and 4 bits assigned to the drop node ID. Hereafter, a procedure for automatically forming the squelch table using the squelch data link will be described, taking a path shown in FIG. 5 as an example.
FIG. 5 shows a diagram illustrating the path for explaining the procedure for automatically forming the squelchtable. In the path shown in FIG. 5, an optical signal is added from the E side of node 2, transmitted to the W→E direction, and passed through node 1. Then, the optical signal is dropped on the W side of node 4.
FIG. 6 shows a diagram illustrating transmission/reception values of the squelch data link in each node. FIG. 6(a) is a diagram illustrating a data link state in the initial state prior to the path setting, in which the entire fields are set to “0”. In FIG. 6(b), first, since node 2 recognizes that the self-node is an add node, node 2 stores the self-node ID (in this case, an absolute ID “2” of node 2) into the specified field (transmission, E side, W→E direction, and SRC_ID) of the data link format, so as to transmit to node 1 as a transmission value. The absolute ID is a proper ID assigned to each node. Here, at this time point, node 2 does not know which node is a drop node, and therefore, the corresponding destination node ID (DST_ID) is left intact at “0”. On the W side of node 1, node 1 receives the transmission value from node 2, as a reception value. By this, node 1 can recognize which node is the add node. Since node 1 is not a drop node, node 1 transmits the reception value received from node 2 intact, from the E side to the W→E direction, as a transmission value of the E side.
Then, as shown in FIG. 6(c), in node 4, the transmission value from node 1 is received as a reception value on the W side of node 4. With this, node 4 can recognize which node is the add node. Further, since node 4 recognizes that the self-node is the drop node, node 4 stores the self-node ID into the specified field (for transmission, W side, W→E direction, and DST_ID) of the data link.
When storing the destination ID, a relative ID being referenced from the source node ID is used, instead of an absolute ID like the aforementioned source node ID. The relative ID is an ID representing the number of nodes ahead from the source node. Therefore, the relative ID varies as the source node varies. In the exemplary case shown in FIG. 5, since node 4, or the drop node, is located two nodes ahead from node 2, or the add node (the source node), “2” is stored as the relative ID. Because each node recognizes the arrangement order (topology) of the nodes constituting the ring network, it is possible to identify any node from the relative ID. Additionally, as to the reason for using a relative ID as destination ID, description has been given in the Patent document 1 (the official gazette of the Japanese Unexamined Patent Publication No. 2002-141924) listed below. In short, the reason is that, by use of the relative ID, it becomes unnecessary to use “0” for identifying the drop node, thereby making it possible to transmit a different kind of information by allotting another meaning to “0”.
Referring back to FIG. 6(c), a data link value representing the relative ID of node 4, the drop node, is transmitted as a transmission value from the W side of node 4, and received on the E side of node 1 as a reception value. Since node 1 is a through node, the above reception value is transmitted intact as a transmission value from the W side, which is then received on the E side of node 2. On receiving the relative ID, node 1 and node 2 can recognize which node is the drop node, based on the received relative ID and the topology information.
Thus, when recognizing the add node and the drop node in the path, each node generates the squelch table.
In the aforementioned squelch data link format shown in FIG. 4, the cross-connect information constituted of the combination of the add node (source node) and the drop node (destination node) is formed of 1 byte: 4 bits assigned to the add node ID, and 4 bits assigned to the drop node ID.
FIG. 7 shows a diagram illustrating the bit assignment of the squelch data link format. As shown in the figure, in the cross-connect information formed of 1 byte, the upper 4 bits are assigned to the source node (SRC_ID), while the lower 4 bits are assigned to the destination node (DST_ID).
As such, in the format of the current state, because the number of bits assigned to the source node and the destination node are 4 bits, respectively, the assignable values are restricted to 16 kinds, 0 to 15. Under such the restriction, the upper limit of the nodes which can exist in one BLSR ring is 16, and it is not possible to accommodate nodes exceeding 16 in one ring.
When it is intended to form a ring network accommodating more than 16 nodes, it can be realized to form a plurality of BLSR rings each having 16 nodes or less, and interconnect the rings. For such the ring interconnection system, DCP (Drop and Continue on Protection), DTP (Dual Transmission on Protection), etc. are known. As to the ring interconnection system, since detailed description has been given in the Patent document 1 (the official gazette of the Japanese Unexamined Patent Publication No. 2002-141924) listed below, of which description will be given later, the description is omitted here.
However, in the trend of large scale networks in recent years, it has been desired to expand the upper limit of the number of nodes accommodable in one ring. Because no means is provided for relieving data in the event of a fault on the line between the rings, in contrast to the protection channels provided in the ring, data relief is intended by means of the aforementioned DCP and DTP connection systems. However, as compared to the intra-ring transmission, fault tolerance performance in the inter-ring transmission is degraded. Therefore, in order to increase the fault tolerance performance also, it is desired to expand the number of nodes accommodable in one ring.
In the Patent document 2 (the official gazette of the Japanese Unexamined Patent Publication No. 2003-224571) listed below, there has been disclosed a technique for enabling 16 nodes or more to be accommodable in one ring. However, the technique concerned is intended to expand the number of nodes which are possible to decide a path switchover in BLSR, and it is not possible to automatically form a squelch table in regard to a ring accommodating more than 16 nodes.
Also, in the Patent document 2 (the official gazette of the Japanese Unexamined Patent Publication No. 2002-141924), although there has been disclosed a method for setting an interring connection system such as the above-mentioned DCP and DTP, the setting method therefor is rather complicated.
Patent document 1: The official gazette of the Japanese Unexamined Patent Publication No. 2002-141924.
Patent document 2: The official gazette of the Japanese Unexamined Patent Publication No. 2003-224571.